1. Field of the Invention
The present invention relates to a technology of suppressing superimposition of magnetic field noise generated from a coil, which is implemented in an imaging apparatus main body or an interchangeable lens, on an imaging element.
2. Description of the Related Art
An imaging element to be mounted in an imaging apparatus such as a digital camcorder or a digital still camera has increased in ISO sensitivity in recent years, thereby being capable of imaging a more clear image even in a scene where a sufficient amount of light is unavailable, such as a night view. However, an increase in sensitivity gives rise to a problem in that small noise, which has not heretofore been a problem, affects the imaging element so as to cause an image disturbance.
For example, in a digital single lens reflex camera, a motor with a coil for driving a lens is provided in an interchangeable lens, and magnetic flux generated from the coil may affect the imaging element to cause the disturbance in the generated image.
In Japanese Patent Application Laid-Open No. 2011-54643, there is disclosed a structure in which a plate-like or film-like conductive member, which is made of a non-magnetic material and has an opening, is provided around the imaging element. When magnetic field noise due to high-frequency electromagnetic waves from the outside, variations in magnetic field, and the like is input into the imaging element, an eddy current flows through the conductive member. In this Japanese Patent Application Laid-Open No. 2011-54643, the magnetic field noise from the outside is cancelled by a demagnetizing field generated by the eddy current, to thereby reduce the magnetic field noise that is input into the imaging element.
In general, a skin depth d caused by a skin effect is expressed by: skin depth d=√(1/(π·f·σ·μ)), where f is a frequency, μ is magnetic permeability, and σ is electric conductivity. When a thickness of the conductive member formed of the non-magnetic material is less than the skin depth d, the thickness is small but the eddy current that flows through the conductive member is insufficient. As a result, the effect of cancelling the magnetic field noise is small, and the magnetic field noise penetrates through the conductive member to be input into the imaging element, which affects the picked-up image. Therefore, in order to allow enough eddy current to flow through the conductive member and in order to obtain the effect of cancelling the magnetic field noise by the demagnetizing field generated by the eddy current, the thickness of the conductive member needs to be the skin depth d or more.
As described above, with the conductive member described in Japanese Patent Application Laid-Open No. 2011-54643, the thickness needs to be the skin depth or more, and because the non-magnetic material has relative magnetic permeability of about 1, the thickness of the conductive member is inevitably increased. Therefore, in order to obtain the effect of cancelling the magnetic field noise by the conductive member while avoiding interference with another member, the imaging apparatus and the interchangeable lens have inevitably increased in size.
In view of the above, the present invention is directed to reducing an amount of arrival by which magnetic flux generated at a coil arrives at an imaging element by a ferromagnetic member having a smaller thickness than before.